1. Field of the Invention
This invention relates to a battery check system for a device having a dual power supply system, e.g., a device which can be supplied with power either from an exclusive battery pack or from an external power source.
2. Description of the Prior Art
In many recent photographic cameras, various control mechanisms, driving mechanisms and the like are electrically driven. Further, there has been developed a photographic camera which has a built-in microcomputer and in which each mechanism is electrically controlled. In such photographic cameras, a predetermined supply voltage must be ensured.
In a large-sized photographic camera, a dedicated battery pack having a large capacity is generally used in place of the dry cell employed as a power source in a normal 35 mm camera in consideration of the large power consumption of the large-sized camera. The battery back generally comprises a plurality of dry cells connected together to obtain a predetermined supply voltage or a secondary battery such as a nickel-cadmium battery. Especially, the nickel-cadmium battery is advantageous as the power source for a photographic camera requiring a large electric current in that it can stably discharge a large electric current and is easier to handle than the battery pack formed of a plurality of dry cells.
Since if the supply voltage is lowered below a certain level, the camera can malfunction, the camera is generally provided with a battery check system for detecting residual capacity of the battery pack so that the time for renewing the battery pack can be easily known.
On the other hand, some cameras are arranged so that power can be supplied from either of two power supply systems, one being a battery pack and the other comprising a D.C. plug connected to an AC/DC converter, so that the camera can be supplied with power via the D.C. plug in a place like a studio where an A.C. power source is available, thereby facilitating the taking of numbers of photographs, continuously.
As the battery check system for the camera having such a dual power supply system, there has been known one which has a battery check circuit common to the two power supply systems.
However, since the battery pack has a small internal resistance, if a battery check is effected under load conditions where the load is not much more than the actual load occuring upon normal photographing, the voltage drop is not sufficient to check the voltage of the battery pack with high accuracy. Accordingly, the residual capacity must be determined on the basis of the voltage drop when a large current load several to ten or so times as large as the actual load is instantaneously applied thereto. Therefore, a large current must be discharged from the battery pack to some extent.
On the other hand, in the case that the camera is supplied with power through the D.C. plug connected to the A.C. source by way of the AC/DC converter, it may be considered that power can be infinitely supplied, and accordingly, battery checking for detecting the residual battery capacity is not necessary. In this case, detection of voltage drop due to failure of the AC/DC converter or the like, i.e., detection of whether or not normal voltage is supplied, is enough. Such detection can be satisfactorily performed by detecting open-circuit voltage under conditions substantially equal to or smaller than the actual load.
Therefore, if the battery check circuit is common, when battery checking is effected for checking the supply voltage through the D.C. plug, there flows a large current which is not smaller than several times as large as the actual load flowing during battery checking of the battery pack and which can exceed the rated output of the AC/DC converter. If the rated output of the AC/DC converter is exceeded, the circuit elements forming the A.D. converter can be damaged, and if the AC/DC converter is provided with an over current protecting circuit, the protecting circuit may operate to interrupt the power supply, thereby preventing the battery check. On the other hand, when the rated output of the AC/DC converter is selected to conform to the over current, the rating of the elements forming the AC/DC converter such as capacitors, transformers and resistors must be unnecessarily high, whereby the elements are enlarged in size and the manufacturing cost is increased.